HBT integrated circuits have found wide acceptance in industry for use in applications as diverse as satellite communication systems, radar systems, cable television systems, optical receivers, etc. Prior art HBTs tend to be Gallium Arsenide (GaAs) devices. With the industrial acceptance of HBTs has come the need for HBT devices which can be made smaller but without sacrificing the gain of the device and for HBT devices which can operate at even higher frequencies than prior art GaAs devices.
HBTs manufactured from an Indium Arsenide/Aluminum Antimonide/Gallium Antimonide (InAs/AlSb/GaSb) system possess a number of advantages over prior art GaAs HBTs. For example, GaSb is an excellent high-frequency p-type base material, having higher hole mobility than presently used base materials, such as GaAs and In.sub.0.53 Ga.sub.0.47 As. It can be p-doped with Si to densities approaching 10.sup.20 /cm.sup.3, which are equal to the highest densities achievable with GaAs and InGaAs (using C as a dopant). Since base resistance is inverse to the product of hole mobility and doping level, the higher mobility translates to lower resistance for p-doped GaSb bases, which will increase the operating frequency limit of a device.
An InAs/AlSb/GaSb material system is also preferable for HBTs being fabricated with submicron feature sizes. Prior art HBTs have relatively low surface Fermi level pinning energy, and as a consequence suffer from recombination of carriers at mesa sidewalls resulting in substantial surface depletion effects. As feature dimensions shrink, these surface effects become proportionally more significant, limiting the size reduction which can be achieved without excessive loss of device performance in terms of the gain of the device. By contrast, the surface pinned energy for GaSb is near the valence band maximum. Accordingly, a p-type GaSb base layer would not have significant surface depletion effects at mesa sidewalls, and could thus be scaled down with less loss of gain due to such surface effects.
In addition to the above advantages, the InAs/AlSb/GaSb material system allows very flexible bandgap engineering. InAs, AlSb, and GaSb have nearly equal lattice constants, such that varying combinations of the materials may be fabricated, in reasonable thicknesses, without suffering serious crystalline defects. Consequently, flexible engineering is possible which will permit implementation of advanced features, such as drift fields in the base material to sweep minority carriers across the base to the collector.
A pnp InAs/AlSb/GaSb structure has been tested, as reported by Pekarik et al., "An AlSb--InAs--AlSb double-heterojunction P-n-P bipolar transistor," J. Vac. Sci. Technol. B, volume 10 no. 2, March/April 1992, pps. 1032-1034. This device does not employ either a GaSb base nor a superlattice in the emitter or collector, and is not of the preferred npn structure. Npn HBT devices are generally preferred by those skilled in the art for high performance applications.
Although desirable, InAs/AlSb/GaSb heterostructure systems have been difficult to fabricate. First, the available emitter materials, AlGaSb or AlSb, require Tellurium (Te) for n-type doping. Tellurium is inconvenient for use in Molecular Beam Epitaxy (MBE) systems, because its memory effects make it difficult to avoid unwanted Te in subsequent layers, and because the Te ties up an available port. Second, AlGaSb and AlSb have a conduction band mismatch with the preferred GaSb base materials. If they are to be used as emitter materials, they need sophisticated grading to deal satisfactorily with the conduction band offset. They are even less desirable as collector materials, because the noted conduction band offset can cause a trapping of carriers.
Accordingly, there is a need for an InAs/AlSb/GaSb HBT structure having a GaSb base, and having improved conduction band alignment across the base-emitter and base-collector junctions. Ideally, such an HBT would be easy to dope. The present invention addresses these needs by employing an InAs/AlSb superlattice which can be constructed to achieve nearly perfect conduction band alignment with GaSb. The entire HBT structure can be doped using only Si for both n-doping of the emitter and collector and for p-doping of the GaSb base. Moreover, the InAs/AlSb superlattice has a valence band offset to GaSb of approximately 0.475 V, enhancing the gain characteristics of devices fabricated according to the present invention.
Si-doped InAs/AlSb superlattices have been used as n-type cladding layers for infrared lasers, as described in U.S. Pat. No. 5,594,750 to Hasenberg and Chow. They have also been used as Schottky barrier layers [Chow, Dunlap, et al., IEEE ElectronDevice Letters, Vol. 17, p. 69 (1996)].